This invention relates to the manufacture of a water degradable sulphur fertilizer and, more particularly, to a process for preparing solid dust-free pellets of such fertilizer and the product formed by such process.
The importance of sulphur as a soil additive, either in the form of sulphur compounds or in its elemental form, with or without a medium such as mineral clay, suspended therein, has been widely understood in the literature for some time. It is important that the elemental sulphur, as a soil additive, be in a finely divided form. In this form it may be stabilized with suspensions therein of certain mineral clays, such as bentonite.
A particularly successful method for preparing such a product is described in U.S. patent application Ser. No. 683,900, filed May 11, 1976, by Bob L. Caldwell, and entitled "GRANULAR SULPHUR-BENTONITE MIXTURE FOR FERTILIZER AND METHOD OF PREPARING SAME." The product manufactured by that process, under the trademark AGRI-SUL, has enjoyed great commercial success. The process described in the aforementioned application comprises the steps of adding one to three parts (out of ten parts) of substantially dry clay dust to seven to nine parts of molten sulphur, with continuous and thorough mixing, at an elevated temperature between 240.degree. and 300.degree. F., pouring the uniform mixture onto a moving wet stainless steel belt, allowing the mixture to cool to a thickness of one-quarter to one-half inch, curing the cooled mixture, comminuting the mixture and screening and separating the granules having a particle size of -6/+16 mesh on the U.S. standard scale. While such a process results in a highly satisfactory product that has become widely recognized as a desirable soil additive, such process does suffer from several drawbacks. First, the product prepared by such process tends to produce a quantity of dust, both during the manufacture of the product and in the end product itself. Second, because of the dust and the subsequent risk of explosion thereof, it is desirable to provide an inert gas atmosphere during manufacture.
It is therefore an object of the present invention to provide an improved manufacturing process for a water degradable sulphur-bentonite mixture.
One method for producing a more desirable product form would be to form solid dust-free pellets, or prills, of the sulphur-bentonite mixture. It is well known that sulphur can be prilled using water as a cooling agent. An apparatus and a process for pelletizing elemental sulphur is described in U.S. patent application Ser. No. 669,023, filed Mar. 22, 1976 now U.S. Pat. No. 4,052,136, Oct. 4, 1977. by Ronald B. Fletcher, and entitled "SULPHUR PELLETIZING PROCESS AND APPARATUS." This last-mentioned patent application discloses a process which includes the steps of forming droplets of molten sulphur and passing the droplets through a liquid cooling medium, preferably water, at a temperature range wherein annealing of the sulphur will occur. Although the solid prills are dust-free and the manufacturing process is also dust-free, since the desired sulphur-bentonite mixture is water degradable, the aforedescribed prilling process and apparatus has been found to be wanting because the sulphur-bentonite mixture does not pelletize in the water, but rather degrades.
It is therefore a further object of this invention to provide an improved process for forming prills from a sulphur-bentonite mixture.